The present invention relates to a process for preparing microcapsules having a wall membrane made of a polyurethane urea resin. More particularly, the present invention relates to a process for preparing microcapsules having a uniform distribution of particle size.
Various processes for producing microcapsules have been known, including the coacervation process, the interfacial polymerization process and the in-situ process. Microcapsules prepared by these processes are widely used in various fields such as medicine, pesticides, dyes, adhesives, liquid fuels, perfumes and liquid crystals.
In these microcapsules, the particle size is an important factor that determines their quality. It is considered desirable to prepare microcapsules having a uniform particle size. In the case of microcapsules used for pressure-sensitive recording sheets, in general, the more uniform the particle size distribution, the better the coloring properties and pressure resistance of the sheet. The reason for this is considered to relate to the fact that microcapsules having considerably smaller particle sizes than the average value do not break upon coloring and thus do not contribute to coloring, while those having too great particle sizes are apt to rupture and thus easily cause so-called pressure fogging.
Thus, it has been important to obtain microcapsules with a desired particle size and a narrow particle size distribution in order to produce excellent pressure-sensitive recording sheets.
For this purpose, there has been proposed a process involving mixing an oily solution and an aqueous solution, pumping the resulting mixed solution through a cylindrical member in which is disposed a static liquid shearing device or a liquid shearing device which moves under the action of the passing solution to form an oil-in-water type emulsion, and then forming a wall membrane thereon to prepare microcapsules. Such a process is disclosed in JP-A-57-84740 (the term "JP-A" as used herein means an "Unexamined Published Japanese Patent Application"). There has also been proposed a process for the preparation of urea-formalin or melamine-formalin resin capsules which includes steps of mixing an oily solution and an aqueous solution, pumping the resulting mixed solution into a spindle through introduction pores opening tangentially at the center thereof while the solution is spirally rotated so that the solution reaches injection pores opening at both ends thereof to form an oil-in-water type emulsion, and then forming a wall membrane thereon to prepare microcapsules. This type of process is disclosed in JP-A-59-87036.
However, none of these processes can provide a particle size distribution sufficiently narrow to drastically improve the properties of the microcapsules since the shearing force acting on the emulsion is nonuniform in the emulsification process (where the particle size of microcapsules is determined). Thus, the particle size distribution can be narrowed only to some limited extent using these processes.
In another example of a known process, an emulsifying device such as a high-shear agitator, a homogenizer or an in-line mixer is used. However, this process can provide only an emulsion with a broad particle size distribution since the region on which the shearing force necessary for emulsification acts is limited to regions very near the emulsification blade, and the shearing force is nonuniform over the distance to the emulsification blade.